This application claims priority and is a divisional of U.S. patent application Ser. No. 10/844,781 filed on May 13, 2004.
The present invention relates to a bottom emission type light-emitting apparatus having a substrate on which a light emitting device is formed. The light-emitting device is an organic electroluminescent device (organic EL device) or an inorganic electroluminescent element (inorganic EL elements). Such a device has a light-emitting layer held between a pair of electrodes. A bottom emission type light-emitting apparatus emits light generated by the light-emitting layer through a substrate. The present invention also pertains to a method for forming the apparatus.
Lighting systems and displays using a light-emitting apparatus such as an organic electroluminescent apparatus (organic EL apparatus) and an inorganic electroluminescent apparatus (inorganic EL apparatus) have been proposed. Such light-emitting apparatuses are roughly divided into a bottom emission type, which emits light generated by a light-emitting layer through a substrate, and a top emission type, which emits light through a side opposite from a substrate.
It has been pointed out that bottom emission type light emitting apparatuses extract relatively small portion of light that is generated by a light-emitting layer.
In a bottom emission type organic EL apparatus shown in FIG. 13, light that enters a transparent substrate 100 is not entirely emitted through a light exit surface 100a of the transparent substrate 100. For example, some of the light reaches the light exit surface 100a at an angle that is greater than a critical angle, which is defined by the transparent substrate 100 and an outside atmosphere (typically, air). Such light includes a portion that is reflected toward an organic EL device 200 and extracted outside the apparatus through an edge as a ray h1, and a portion that attenuates in the apparatus as a ray h2. These portions of light cannot be utilized.
Conventionally, techniques to cope with the above drawbacks have been disclosed (for example, Japanese Laid-Open Patent Publication No. 9-129375). Specifically, the surface of a transparent substrate at the light exit side (light extraction face) is formed as a diffuse surface (uneven face having asperities) so that the containment of light due to total reflection at the light extraction face is decreased. The extraction efficiency is improved.
However, in some cases, forming asperities on the light extraction face practically does not improve the extraction efficiency compared to a case where no uneven face is provided.
Without increasing the amount of light emitted in a specific direction from the light exit surface, light extracted from a light emitting apparatus substantially cannot be used.